The frontopolar cortex (FPC) contributes to tracking the reward of alternative choices during decision making, as well as their reliability. Whether this FPC function extends to reward gradients ...associated with continuous movements during motor learning remains unknown. We used anodal transcranial direct current stimulation (tDCS) over the right FPC to investigate its role in reward-based motor learning. Nineteen healthy human participants practiced novel sequences of finger movements on a digital piano with corresponding auditory feedback. Their aim was to use trialwise reward feedback to discover a hidden performance goal along a continuous dimension: timing. We additionally modulated the contralateral motor cortex (left M1) activity, and included a control sham stimulation. Right FPC-tDCS led to faster learning compared to lM1-tDCS and sham through regulation of motor variability. Bayesian computational modelling revealed that in all stimulation protocols, an increase in the trialwise expectation of reward was followed by greater exploitation, as shown previously. Yet, this association was weaker in lM1-tDCS suggesting a less efficient learning strategy. The effects of frontopolar stimulation were dissociated from those induced by lM1-tDCS and sham, as motor exploration was more sensitive to inferred changes in the reward tendency (volatility). The findings suggest that rFPC-tDCS increases the sensitivity of motor exploration to updates in reward volatility, accelerating reward-based motor learning.
A growing literature points to a specific role of the cerebellum in affect processing. However, understanding of affect processing disturbances following discrete cerebellar lesions is limited. We ...administered the Tübingen Affect Battery to assess recognition of emotional facial expression and emotional prosody in 15 patients with a cerebellar infarction and 10 age-matched controls. On emotional facial expression tasks, patients compared to controls showed impaired selection and matching of facial affect. On prosody tasks, patients showed marked impairments in naming affect and discriminating incongruencies. These deficits were more pronounced for negative affects. Our results confirm a significant role of the cerebellum in processing emotional recognition, a component of social cognition.
Performance of a unimanual motor task often induces involuntary mirror electromyographic (EMG) activity in the opposite, resting hand. In spite of the ubiquitous presence of mirroring, little is ...known regarding the underlying cortical contributions. Here, we used functional magnetic resonance imaging (fMRI) to study brain regions activated in association with parametric increases in right isometric wrist flexion force (10%, 20%, 30%, and 70%) in 12 healthy volunteers. During scanning, EMG activity was recorded bilaterally from flexor carpi radialis (FCR), extensor carpi radialis (ECR), biceps brachii (BB), and triceps brachii (TB). Mirror EMG was observed in left FCR during 20%, 30%, and 70% of force. Left ECR, BB, and TB showed mirror EMG only at 70% of force. Increasing force was associated with a linear increase of blood-oxygen-level–dependent (BOLD) signal in bilateral primary motor cortex (M1), supplementary motor area (SMA), caudal cingulate, and cerebellum. Mirroring in the left FCR correlated with activity in bilateral M1, SMA, and the cerebellum. Overall, our results suggest that activity in these regions might reflect sensorimotor processes operating in association with mirroring and suggest caution when interpreting fMRI activity in studies that involve unilateral force generation tasks in the absence of simultaneous bilateral EMG/kinematics measurements.
Cortical activity during simple unimanual actions is typically lateralized to contralateral sensorimotor areas, while a more bilateral pattern is observed with an increase in task demands. In ...parallel, increasing task demands are associated with subtle mirror muscle activity in the resting hand, implying a relative loss in motor selectivity. The corpus callosum (CC) is crucially involved in unimanual tasks by mediating both facilitatory and inhibitory interactions between bilateral motor cortical systems, but its association with mirror motor activity is yet unknown. Here we used diffusion-weighted imaging (DWI) and bilateral EMG measurements during a unimanual task to investigate potential relationships between white matter microstructure of the CC and mirror EMG activity. Participants performed a unimanual pinch force task with both hands. 4 parametrically increasing force levels were exerted while electromyographic (EMG) activity was recorded bilaterally from first dorsal interosseus muscles. Consistent with previous findings, mirror EMG activity increased as a function of force. Additionally, there was a significant relationship between the slope of mirror EMG increases and fractional anisotropy in transcallosal fibres connecting both primary motor cortices. No significant relationships were found for fibres connecting dorsal premotor cortices or supplementary motor area, indicating the local specificity of the observed brain–physiology relationship.
Introduction tDCS over the primary sensorimotor cortex (SM1) has been shown to induce changes in motor performance and learning. Recent studies indicate that tDCS is capable of modulating neural ...network properties within the whole brain. Objectives To investigate the temporal evolution of online tDCS effects on functional connectivity within and between the stimulated sensorimotor cortices. Materials and methods Two different tDCS montages were investigated: (i) unilateral tDCS (anode over right SM1, cathode over contralateral supraorbital region) and (ii) bilateral tDCS (anode over right and cathode over left SM1). In a randomized single-blinded crossover design, 12 healthy subjects underwent functional magnetic resonance imaging (fMRI) at rest before, during and after bilateral, unilateral or sham tDCS at rest. Seed-based analysis was used to investigate tDCS-induced changes in functional connectivity between SM1 and interconnected areas. Results Both uni-and bilateral tDCS, induced dynamic and non-linear changes in functional connectivity of both SM1 and interconnected brain areas. More specifically, tDCS induced decreases in functional connectivity between both SM1 as compared to sham in both conditions. This effect was more prominent during bilateral tDCS as compared to unilateral tDCS. Furthermore, only during bilateral tDCS, an increase in intracortical connectivity within right M1 was observed. Conclusion Our results provide evidence that depending on the electrode montage, tDCS acts upon a modulation of either intracortical and/or interhemispheric processing of SM1.
Introduction Ongoing oscillations are associated with brain functions such as somatosensory perception. For example, the amplitude of the sensorimotor mu rhythm can be linked to the perception of ...near-detection-threshold somatosensory stimuli ( Linkenkaer-Hansen et al., 2004 ). Furthermore the phase of neuronal oscillations affects the perception of near-threshold stimuli ( Busch et al., 2009 ). Transcranial alternating current stimulation (tACS) may offer the possibility to modulate oscillatory activity. Recently it was shown that tACS increased the amplitude of visual alpha oscillations ( Zaehle et al., 2010 ) and had a phase dependent influence on auditory perception ( Neuling et al., 2012 ). Objectives We examined the effect of tACS applied at participants’ individual mu frequency on threshold levels of somatosensory perception. We hypothesized that (a) tACS modulates somatosensory perception thresholds as compared to sham and (b) perception thresholds vary as a function of the phase of tACS. Methods In a randomized, single-blinded, crossover design, 17 participants (mean age: 27; female: 10) underwent a combined EEG/tACS experiment in two separate sessions (real or sham tACS). In the beginning, subject’s individual mu frequency was derived from the event-related desynchronization over the left somatosensory cortex (S1) induced by electric pulses to the right index finger ( Fig. 1 b). Subsequently, somatosensory detection thresholds were determined in a block of 16 min using an adaptive staircase procedure of weak electric stimuli that were presented with electrodes at the right index finger. During the second third of the task 5 min of tACS was applied at the individual mu frequency in a bilateral montage over both primary somatosensory cortices (S1). For sham, 30 s of 1 mA random noise stimulation was applied ( Fig. 1 a). Behavioral performance was assessed with respect to (i) an average effect of tACS as compared to sham and (ii) a modulation dependent on the tACS phase. Results No differences in the average somatosensory perception thresholds were observed between real and sham stimulation. However, during tACS, somatosensory detection thresholds changed as a function of the phase of tACS. Thresholds were differing maximally for stimuli presented at opposite phases in both maxima of the tACS signal curve( Fig. 2 ). Conclusion We conclude that tACS applied at the individual mu frequency over S1 is capable of modulating perception of near-threshold somatosensory stimuli in a phase-dependent manner. Our findings suggest that functionally relevant intrinsic oscillations may be modulated using non-invasive brain stimulation.
Introduction Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique used to modulate cortical excitability. In the motor domain, there is consensus that anodal ...tDCS increases cortical excitability, whereas cathodal tDCS decreases it. On the other hand, inconsistent results of tDCS-induced effects in the sensory domain have been reported. Objective The aim of the present study was to investigate (A) changes in cortical excitability within primary somatosensory cortex (S1) by means of single-pulse somatosensory evoked potentials (SEPs) and (B) intracortical inhibition by means of paired-pulse SEPs when tDCS was applied over left SM1. Materials and Methods 10 min of anodal, cathodal or sham tDCS was applied over the left SM1 using saline-soaked sponge electrodes (35 cm2 electrodes, 1 mA, current density 0.028 mA/cm2 ). Before, immediately after as well as 10 min after termination of tDCS, single-and paired-pulse SEP recordings were performed. We hypothesized that tDCS will induce polarity specific changes in cortical excitability within left S1. Furthermore, we reasoned that anodal tDCS will reduce paired-pulse inhibition within left S1 while cathodal tDCS will result in an augmentation of inhibition relative to sham stimulation. Results 10 min of anodal and cathodal tDCS over SM1 did not result in any significant excitability changes within left S1. However, anodal tDCS resulted in a reduction of paired-pulse inhibition within left S1 10 min after termination of stimulation. No change in paired-pulse inhibition could be observed after cathodal tDCS. Conclusion Here we provide novel evidence that anodal tDCS affects inhibitory processing within S1, a finding that might improve our understanding about the underlying neural mechanisms of tDCS on somatosensory processing.
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